Changes in the expression and function of transport proteins within the blood-brain barrier (BBB) are known to occur during the natural aging process. In the present proposal, the effects of age on the expression and function of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP) in the brain microvessel endothelial cells that form the BBB will be evaluated. Both P-gp and MRP are efflux transport proteins that actively transport a wide variety of drugs and macromolecules out of the cell. The development of drug resistance in cancer cells has been attributed to over expression of P-gp and MRP. The expression of these same efflux transport proteins in normal cells, such as the brain microvessel endothelial cells that form the BBB, are believed to play a protective role, preventing the accumulation of potentially toxic compounds in the brain. The hypothesis of the current proposal is that the expression and/or functional activity of P-gp and MRP in the BBB changes as a function of age. Using brain microvessel endothelial cells harvested from female Fisher 344 rats at 3, and 18 months of age, the specific aims of the proposal are to: 1) examine age-related differences in the expression of P-gp and MRP in freshly isolated brain microvessel endothelial cells and primary cultured rat BMEC monolayers; and 2) examine age-related differences in the cellular accumulation and transcellular permeability of P-gp and MRP probes in confluent rat BMEC monolayers. Expression of P-gp and MRP will be evaluated at both the protein and RNA level using quantitative immunoblot and RT-PCR techniques, respectively. Functional activity of P-gp and MRP in the BBB will be assessed by examining the cellular accumulation and bi-directional differences in the permeability of selected P-gp and MRP probes in primary rat BMEC monolayers. Given the putative protective role of these two drug efflux transporters in the BBB, age- related alterations in the expression or function of P-gp and MRP could lead to increased susceptibility to endogenous factors and xenobiotics that impair central nervous system function.